Roots serve as support for preventing a plant from falling down, and more importantly, draw water and nutrients. A root branches to extend surface area so as to increase an area capable of absorption. The root also has a function of secreting and linking an organic acid, which makes a substance more absorbable, to phosphor, iron, or the like, and absorbing them. The root also has a function of synthesizing and secreting a substance like a lubricant so as to help the tip of the root grow in the soil. Synthesis and storage of plant hormones are also a role of the root. Further, the root stores energy obtained by photosynthesis, or stores phosphor during a juvenile stage and translocates it in the future. Plants, and especially roots, which cannot move as can animals suffer from a number of stresses from the surrounding environment, and take a number of measures against them.
These functions of roots are associated, for example in rice, with the following genes: cyclophyllin (Cyp) gene (Buchholz et al., Plant Mol. Biol. 25: 837-843 (1994)), lipid transfer protein (LTP) gene (Vignols et al., Gene 16: 265-270 (1994)), phenylalanine ammonia lyase (PAL-ZB8) gene (Zhu et al., Plant Mol. Biol. 29: 535-550 (1995)), histone H3 gene (Terada et al., Plant Mol. Biol. 27: 17-26 (1995)), basic chitinase (RC24) gene (Xu et al., Plant Mol. Biol. 30: 387-401 (1996)), caleosin gene (Naested et al., Plant Mol. Biol. 44: 463-476 (2000)), and the like, which are already known to be expressed in the root.
In some plants other than rice, the elongation factor eEF1A gene (Tremousaygue et al., Plant J 20:553-561(1999), myrosinase gene Pyk10 (Nitz et al., Plant Science 161: 337-346(2001)), iron transporter (IRT2) gene (Vert et al., Plant J 26:181-189 (2001)), and the like of Arabidopsis; the glutathione S transferase (GST) gene (Klinedinst et al., Plant Mol Biol 42:679-688 (2000)), nitrate reductase (Hansch et al., J Exp Bot 52:1251-1258 (2001)) and the like of tobacco; or the pectinmethylesterase (Roger et al., Plant Science 160:713-721(2001)) and the like of flax, are already known to be expressed in roots.
Shoot apex refers to the tip of a stem in a nutrition growth stage and its surrounding portions. In higher plants, stem apices have a capability of undergoing cell division to produce new cells. Examples of a gene expressed in stem apices include the lipid transfer protein (Itp1) gene (Canevascini et al., Plant Physiol., 112: 513-524 (1996)); homeobox (NTH15) gene (Tamaoki et al., Plant Cell Physiol., 38:917-927 (1997)), and gibberellin 3beta-hydroxylase gene (Itoh et al., Plant J., 20:15-24 (1999)) of tobacco (Nicotiana tabacum); and the pyrroline-5-carboxylate reductase gene (Hua et al., Plant Physiol., 114: 1215-1224 (1997)), ERECTA gene (Yokoyama et al., Plant J., 15:301-310 (1998)), ATML1 gene (Sessions et al., Plant J., 20:259-263 (1999)), and FAD3 gene (Matsuda et al., Planta, 213:833-840 (2001)) of Arabidopsis. These studies demonstrated that the tissue specificity of expression of these genes is regulated by their promoters.
Recently, a method of linking a promoter capable of expressing in roots with a gene capable of contributing to the development, disease resistance and stress resistance of a root and introducing the gene into plants has received attention as a promising biotechnology technique.
An attempt has been made to use a promoter for a viral gene in order to express a foreign gene in plants (e.g., MacFarlane and Popovich, Virology 267: 29-35 (2000); and Mazithulela et al., Plant Science 155: 21-29 (2000)). However, viruses spread after infection. Promoters derived from plant genes are superior to promoters derived from viral genes for the purpose of limiting the expression site of genes to a cellular level.
There are several known promoters for genes expressed in roots, which have been found in maizes and the like. Recombinant DNA technology can be used to confer disease resistance in roots. U.S. Pat. No. 6,284,948, entitled “Genes and methods for control of nematodes in plants”, discloses an example of conferring nematode resistance in roots. Here, a nematode resistance gene is expressed using a plant ubiquitin gene promoter. However, this ubiquitin promoter has a low level of organ specificity and is constantly expressed.
An example of a root-specific promoter is disclosed in U.S. Pat. No. 6,271,437, entitled “Soybean gene promoters”. Here, a method is described in which a promoter for a soybean cyst nematode gene is used to express a foreign gene in roots. Examples of a toot-specific promoter are disclosed in USP application 2001/0016954, entitled “Root specific promoters”. These examples include a b1-tubulin gene promoter derived from Arabidopsis, a ribosome protein RPL16A gene promoter, an ARSK1 gene promoter, and a soybean metallothionein-like gene promoter. It is described that these promoters were used to confer nematode resistance.
These promoters have the following problems: the activity of the promoters is generally insufficient to provide a practical application; the promoters do not function in monocotyledons, including cereals such as rice and the like; their organ specificity is low; or the like.
Therefore, if a number of promoters acting in various development stages of cereals are isolated, the features of these promoters are revealed, and promoter cassettes having different tissue specificities and high activity are produced, then such promoter cassettes are very useful for breeding crops, such as rice and the like.